Well, Mickey, it's hard to make suggestions without knowing a lot more about your set-up but a few things come to mind.
Guppies and plecos may be a girl's best friend, but Bettas are the odd-man-out in South American set-ups. I'd take that Betta out of there. Bettas just cause trouble, IMO, in community tanks, and females can be just as bad as males! Bettas do like confined spaces, and I am not surprised that yours likes the breeder trap. I would not put her back in with the guppies, unless you want them to be eaten. I am sure that she was harrassing the whiptail, too. You just didn't see it.
I know you're doing the best with what you have got, but I would have said that a 10 gallon tank is too small for a
Rineloricaria species catfish. Even though they are long and skinny, whiptails need the length and water volume of a larger tank. A 20 G long is a good size. It's hard to keep up water quality in 10 G, although you seem to be on target with water changes. Is your bottom material clean and aerated? Could your whiptail have eaten spoiled food?
My whiptail roots around on the sand a lot and chugs along like a jerky lawnmower, if you can envision that. He can be seen on the glass, too, but I don't know what he likes about it. Maybe he is grazing algae sometimes. All I know is, he doesn't sit around not eating. However, sometimes he justs rests under a branch and looks like he's hardly breathing. That's normal.
Remember that your whiptail is not a vegetarian. His regular diet should have been a mixture of things. I'm suggesting the vegetable now, because he seems lethargic and blocked up with something. Possibly he is just stuffed, and is taking a break. If he doesn't feel good, then you are doing some things to make him feel better, and I don't know what else you can do until you see another symptom.
So, to recap, here are my suggestions:
Remove the Betta.
Fast the whiptail one or two days.
Offer a cooked veggie as the next food.
Do the water changes. Fry need to have clean water, too, so I'm sure they'll be fine. Just be careful not to remove the babies with the bathwater!
If he hates the veggie, offer Omega One Kelp Flakes or something like that, instead.
Keep us posted on how he's doing.
Deborah
BTW, I'm not suggesting an Epsom Salts bath because I don't know how you'd do that with the baby guppies in there. It's kind of old-fashioned, but some aquarists give these baths to constipated fish. Since it's not clear what's wrong with your fish, I can't suggest this route.